Spectral radius of the iteration matrix ρ(G) for multiple formal solvers and for the damped-Jacobi and SOR iterative methods with N_Ω = 20, N_ν = 21, and varying N_s. For the damped-Jacobi and SOR methods, we used ω_opt from Eq. (11) and ω_sor from Eq. (12), respectively. Since Eq. (12) cannot be applied if κ < 1, the ω_sor corresponding to DELOPAR is used for the DELO-parabolic formal solver.